Electrophotographic marking is a commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a substantially uniformly charged photoreceptor with a light image representation of a desired document. In response, the photoreceptor discharges, creating an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto that latent image, forming a toner image. That toner image is then transferred from the photoreceptor onto a substrate such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure, thereby creating a copy of the desired image. The surface of the photoreceptor is then cleaned and recharged for the production of another image.
One way of exposing the photoreceptor is to use a Raster Output Scanner (ROS). A ROS is comprised of a laser light source (or sources) and a rotating polygon having a plurality of mirrored facets. The light source radiates a laser beam onto the polygon facets. That beam reflects from the facets and strikes the photoreceptor, producing a light spot. As the polygon rotates the spot traces lines, referred to as scan lines, on the photoreceptor. By moving the photoreceptor as the polygon rotates, the photoreceptor is raster scanned by the spot. During scanning, the laser beam is modulated according to image information so to produce the desired latent image.
The foregoing broadly describes the operation of an electrophotographic marking machine. Such marking machines are often used in digital copiers. In addition to an electrophotographic marking machine, digital copiers include an input scanner that scans an image on an original document so as to produce a digital representation of that image. Then, the electrophotographic marking machine reproduces that image, possibly after being modified according to one or more user requirements.
Digital input scanners typically employ one or more arrays of photosensitive elements, such as charge coupled devices ("CCDs"), an illumination lamp, and an optical system. The lamp illuminates an original document and the optical system focuses light reflected from the illuminated document onto the photosensitive elements. Since the reflected light intensity varies according to the image on the document, the photosensitive elements are able to convert the reflected light into electrical signals that represent the document. Typically, the document is scanned as a sequence of line images, with each line image being referred to as a scan line. Thus, two uses are made of the term--scan line--, one referring to lines that are produced by sweeping the laser beam spot across the photoreceptor, the other referring to a line image scanned by the input scanner.
Conventionally, each input scan line is imaged during an integration period of a predetermined duration. During the integration period the photosensitive elements produce charges that are proportionate with the intensity of their input light. Those charges are accumulated on a capacitor. At the end of the integration period, the stored charges result in a potential that is then digitized to represent the charge buildup. The duration of the integration period, which must be sufficiently long to fully integrate the image line being scanned, yet not so long as to allow the photosensitive elements to saturate, is set by a fixed rate clock signal in the prior art.
It can be seen that input scanners and raster output scanners are both characterized by scan lines. An input scanner might scan 400 lines per inch while a raster output scanner might write 1200 lines per inch. In the prior art, the input scanner scan rate is set by an oscillator that is derived from a crystal source (possibly via a microprocessor timer that is programmed to slightly vary the oscillator frequency). However, because the output scan rate depends upon the mechanical rotation of a polygon, the output scan rate is difficult to control accurately. In the prior art the resulting variations between a highly stable input scan rate and a far less stable output scan rate typically did not matter because the digitized input image is usually stored in a "buffer memory." The input image data is then read from the buffer memory as required. Essentially, in the prior art the input scanner data flow is synchronized with polygon motion.
While the prior art scheme of handling polygon motion variations is successful, it is not suitable for highly cost sensitive digital copiers. Not only must a buffer memory and an input scanner oscillator be provided, but also synchronization circuitry between the buffer memory and the polygon is required. Otherwise, variations in polygon rotational velocity (which might be +/-0.1% of nominal) would result in "slow scan magnification errors." The slow scan direction is the direction perpendicular to the scan line traced by the spot and is brought about by the motion of the photoreceptor. Basically, if the polygon is rotating too slow the photoreceptor motion would cause the image to magnified in the slow scan direction, if the polygon is rotating too fast, the photoreceptor motion would cause the image to be reduced in the slow scan direction.
Because scan line magnification errors are detrimental to print quality a technique of reducing or eliminating such errors would be beneficial. Even more beneficial would be a technique of reducing or eliminating scan line magnification errors that is suitable for cost sensitive machines.